Overload clutches are used to protect structural components against an overload, thereby avoiding an interruption of an operational sequence. Without such protection an interruption can occur when a structural component in a train of movement or power transmission train suddenly fails due to an overload, whereby a jamming of the operation or motion could be caused.
There are many areas in which such a jamming is very undesirable. For example, an aircraft door must be operable with certainty. For opening an aircraft door, the locking mechanism of the air-craft door is constructed for first lifting the door out of a locked position and to then tilt the door outwardly. In an emergency the lifting mechanism of the door operates a lever in the door frame by means of a bolt forming part of the door. This lever is connected to the end of a shaft by means of an overload clutch for deploying emergency slides or chutes. An opening motion of the air-craft door in an emergency is transmitted through the overload clutch and through a Bowden pull which applies a linear motion to the emergency slide release mechanism. As a result, in an emergency the opening of the door automatically deploys the emergency chute into an operational condition so that passengers may slide down the emergency chute. For a proper and safe operation of the emergency chute release mechanism, it is necessary that the opening motion of the door is transmitted through the overload clutch to the Bowden pull without any slippage. However, if in an emergency the Bowden pull should be blocked, this fact must not prevent the opening of the door. For this purpose, it is necessary that the overload clutch will yield upon reaching of a determined torque moment, so that the door can be properly opened in spite of the blocked Bowden pull.
A so-called ball safety clutch of conventional construction cannot be used in the above described situation, because such a clutch does not assure a slip-free power transmission up to a determined torque load. In a ball safety clutch, the drive is connected to the clutch housing, while a power take-off is provided with an entraining cam disk having several radial recesses Both, the drive and the entraining cam disk are rotatably mounted to the housing. The housing also supports a cage element comprising a plurality of axial bores corresponding to the number of recesses in the entraining cam disk. The cage element is positioned opposite to the recesses in the entraining cam disk. Each axial bore holds a ball biased by a compression spring for engaging one of the recesses in the entraining cam disk. An adjustable pressure plate is also provided on the housing in such a way that the balls are biased with a selectable spring force into the depressions of the entraining cam disk. In this type of structure, the occurrence of any torque moment at the ball safety clutch causes a rotation of the drive components relative to the power take-off components so that a slip-free transmission of the motion is not possible in the just described conventional slip clutch even before the above mentioned determined torque load is applied to the input of the clutch.